Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10~7~78
.
This invention relates to a relatively simple efficientand economical process for removing particulates and gases such
as sulfur oxides, hydrogen sulfide and organic sulfur compounds
from an industrial gas stream. ~ixed emissions of this type
are commonly found, for example, in Kraft and sulfite recovery
processes in the pulp and paper industries. Prior art processes
teach various methods of removing these types of emissions
individually, however, none of the prior art teaches an econo-
mical coordinated process for the removal of all of these compo-
nents. Furthermore, in some cases, a prior art process for the
removal of one component interferes with or reduces the efficiency
of subsequent removal steps for other components.
For e~ample, emissions from Kraft recovery boilers
typically consist of hydrogen sulfide and organic sulfur compounds
(designated "TRS" for total reduced sulfur), S02 and ~articulates.
The organic su]fur compounds typically consist of mercaptans such
as methyl mercaptan (C113SH), mercapto ethers such as dimethyl
sulfide (CH3SCH3), and disulfides such as dimethyl disulfide
(CH3S SCH3). Some references indicate the presence of carbonyl
sulfide (COS). The quan~ity and composition of emissions are a
function of boiler feed and loading, boiler operation, and
proce su]~idity.
` . - .
~ ~ p~qe 3
1~:;7678
Emissions from boilers are g~nerally in the broad
range of:
TRS: 10-2500 PPM (parts per million)
Particuiatcs: 1-7 ~r/sdcf ~grains per standard
dry cubic foot)
S2: 10-200 PPM (parts per million)
The permissible emissions from recovery boilers are,
increasingly, being restricted by government authorities.
Although the level of restriction varies with the specific
authority, the emerging standards for 1977 appear to be TRS
less than 5 PPM and particulates less than 0.08 to 0.04 gr/sdcf.
In some new boiler designs, TRS emissions can be
controlled to 3-10 PPM when operating at 80-100% of design
capacity, but only with close combustion control and decreased
thermal efficiency. Also, particulate emissions present moré
of a problem with this type of design. Black liquor oxidation
processes in combination with existlng furnaces can, with close
control, maintain TRS emissions at 4-30 PPM when operating at
80-100% of design capacity, but the particulate emissions problem
still exists. Electrostatic precipitators in existing recovery
boilers, after an extended period of operation such as 3-5
years, are reducing particulate emissions tO levels OL O.10- .
0.25 gr/sdcf at 80-100% of design capacity. When the boilers
are operated at 120% of design capacity, however, the parti-
¦ culate emissions level in many cases increases to more than
1 gr/sdcf. None of these systems can readily accommodate
fluctuating boiler load levels. Furthermore, electrostatic
precipitators in themselves do not control TRS emissions.
'7~ ~
I`here~fore, it appears tha-t neither elec-trostatic precipitators
alone, black liquor oxidation alone, nor a combination of these
two wel~evaluated systems, are consistently capable of meeting
the overall environmental regulations.
Recently, experimental work has been conducted on the
absorption of sulfur oxides and other sulfur compounds in alkaline
slurries of activated carbon. In particular, U.S. Patent Nos.
3,701,824; 2,823,766; 3,486,852; and 3,824,163 teach that water
slurries of activated carbon can be used to scrub sulfur dioxide,
hydrogen sulfide, and organic sulfur compounds such as mercaptans
and alkyl sulfides from a gas stream. These patents appear to
depend on a combination of sorption and oxidation processes. In
general, these patents teach a carbon slurry concentration of
about 0.1-10% by weight or higher for the cocurrent or counter-
current scrubbing of sulfurous gases having hydrogen sulfide or
organic sulfur compound concentrations on the order of 100-5000
PPM. These patents do not discuss the problem of the removal of
particulates.
Other prior art patents disclosing alkaline scrubbing
reactions are U.S. Patent Nos. 3,852,408; 3,852,409 and 3,755,990.
; U.S. Patent No. 3,324,630 teaches a process for removal
of particulates from a gas stream which utilizes a crossflow
scrubbing technique,
The process disclosed is capable
of removing very small particulates on the order of 0.1-10 microns
in size.
In the U.S. Patent No. 3,957,464 issued May 18,
1976, an improvement in scrubbing is disclosed wherein
the particulate-laden gas stream is first treated under substan-
tially adiabatic conditions to increase its turbulence and to
increase its humidity substantially to saturation at a temperature
:........... . ..
~agc 5
li 10~7~78
~1 ," ,
above about ~50F to initiate nucleation of sn~all particulates
by condensatiol) and/or agglomeration. ~hcreafter the ~as is
contacted with a scrubbing liquor-which can be recirculated
through a packed enclosure, usually at a substantially constant
temperature. This improvement normally eliminates the need for
cooling the recirculating liquor at a savinq in material and
I energy costs.
¦¦OBJECTS OF TI~E INVENTION .
Accordingly, it is a primary object of the present
invention to provide a coordinated and economic process for the ~-
removal of particulates and acid qases from a hot effluent
Igas stream.
¦ It is specifically an object of this invention to
l provide a process for scrubbing particulates, sulfur dioxide,
¦ hydrogen sulfide and organic sulfur compounds from a gas stream
with an aqueous alkaline carbon slurry in a process which requires
¦!a lesser concentration of carbon than has heretofore been possible .
¦ It is also an object of this invention to provide
¦¦a wet scrubbing process for the removal of particulates which
¦ does not normally require the cooling of recycled scrubbing
. ¦ liquor.
It is further an object of this invention to provide
~ a process for scrubbing particulates and sulfurous gases from
;' la gas stream as described wherein efficient removal is obtained
¦ at a minimum caustic and carbon consumption, witll reduced re-
quirements for heat and power and with a lower initial cost of
~equipment.
These and other objects of the invention will become
apparent from the following description.
I a
page 6
¦! ~0~7~7~3 .
BRIEF l)l~:SCRIPTION Ol;' Tlll~ DR~WINGS
FIG. 1 is a flow sheet illustrating the evaporation
and recovcry boilcr portions of a typical pulping process
¦producing an effluent flue gas containin~ particulates and
¦ sulfur-containin~ gases;
¦ FIG. 2 is a schematie view of one embodiment of gas
treatment apparatus of this invention;
FIG. 3 is a partially-cutaway perspective view which
illustrates the structure of one form of the appara~us shown
sehematically in FIG. 2;
FIG. 4 is a graph comparing the efficiency of TRS
¦~removal by laminar eontact serubbin~ with that by a turbulent
contaetor;
FIG. 5 is a graph comparing the efficieney of parti-
eulate removal in the process of this invention at different gas
stream dew point temperatures; and
¦ FIG 6 is a schematic view of a further embodiment of
¦ gas treatment apparatus aeeording to this invention illustrating
¦optional additional treatment steps and apparatus.
2 0 ¦ . FURTHER DESCRIPTION OF THE DRAWINGS
FIG. 1 sehematieally illustrates one type of reeovery
Iboiler operation as employed in pulp manufacture. The liquid
,containing sulfur eompounds and eellulose-lignin organie materials
¦ealled "black liquor", from a digester (not shown) is fed into a
black liquor oxidation chamber 10 where it is exposed to oxygen.
The oxidized black liquor is then fed to a stream-heated evapora-
tor 12 and a direct contact evaporator 14 where water is evapo-
rated to concentrate organic material to combustible levels. The
eoneentrated black liquor is then sprayed into a reeovery boiler
16 where the organic material is burned to recover heat and
I chemieals. The hot effluent exhaust gases, treatment of which
~lis one o~)jec~ o~ e pres~nt invention, ~re then ~ed baek
1, ~
~I
pD~e 7
1 ~067~78
! to evaporator 1~ to recover heat, and thcnce through an electro-
st~tic preci~ or l8 to rcmove pa~ticul.ltes. The ~as exiting
the elcctrostatic precipitator (not always em~loyed) contains
l particula~es, SO2 and TRS, principally hydrogen sulfide but also
S ¦ frequcntly containin~ organic sulfur coMpounds as hereinbefore
described.
Referring now to FIG. 2, the hot inlet qas stream
Si is typically at a temperature of about 300-500F and a dew
l point of about 150-185~F. The gas stream may have previously
¦ been treated for preliminary particle removal by conventional
I ¦ methods discussed hereinafter. The stream Si is directed by
î m~ans of a washed fan 100 at a velocity of about 50 fps into a
¦ venturi 101. The gas is subjected to a liquid spray quench 102
I prior to and/or simultaneously with reaching the venturi throat
¦ 103. A plug 104 having an essentially diamond-shaped cross-
section may be inserted in the venturi throat and has been found
to improve the efficiency of recovery. Venturi 101 is operated
iat a lower pressure drop of the gas therethrough than more
conventional venturis heretofore employed to remove particulates.
The pressure drop of the gas therethrough is less than 20 and
preferably less than about 10 inches of water. In particular,
; I the use of a venturi with a diamond-shaped plug as shown has
been found to faeilitate the removal of intermediate-sized
¦ particles larger than about 0.8 microns at this stage of the
process, and such partieles drop out of the gas stream either
by action of qravity or by impinqinq contact with the spray
formed in the venturi throat 103. The captured particles form
¦ D slurry in tl quench liquor, or, if soluble, dissolve ~hcrein.
i~7~7~
I`he turblllent gas stream S, cooled but still at a
temperature above 150 F and moisturized to near saturation
by the action of the liquid quench, is next channeled through
a set of baffles 105 which are continuously washed by a wash
liquor from nozzles 106. The wash liquor is drained to the
bottom of the apparatus where the solids may be separated by
conventional means such as screen or settling tank means or
left to f`orm a slurry. The wash liquor is combined with the
liquor from the venturi in sump 108 and is recirculated by
pump 109.
Emerging from the baffle system, the gas is substan-
tially saturated with water vapor at a temperature of at least
about 150F to 212 F and nucleation of sub-micron particles
occurs. It should be noted that the increase in turbulence and
saturation of the gas within the enclosure defined by venturi
101, baffles 105 and the walls of housing 107 occurs under
substantially adiabatic or isoenthalpic conditions. No
significant heat is added to or withdrawn from the gas, the heat
of the gas being employed to vaporize the small amount of
moisture required and the vaporization cooling the gas by
lowering its dry bulb temperature. Under equilibrium operation,
with recirculating quench and wash liquor, the temperature of
the liquor and gas will be near the wet bulb temperature of
the incoming gas.
The gas together with the entrained, nucleated particles
is then passed in an essentially horizontal path through scrubber
bed 111, packed with any suitable packing material, preferably
the packing material disclosed in U.S. Patent No. 2,867,425,
also described in U.S. Patent No. 3,324,630, and available
page 9
1~ti7ti78
commcrcially unc~er the trademark "1lellcrettes", more fully
describcd hereinafter, whcre it is brouc3ht into crossflow
! contact witl~ ~I)e scrubbing liquor which is continuously sprayed
, into scr~hhirlg section 111 by nozzlcs 112, 113 and 114. Al-
though FIG. 2 S~IOWS a singlc scrubbinq section with three sets
of nozzles, the number of sections, the size of the sections,
and the number of nozzles per section ls not critical and may
¦¦ be varied to suit individual process requirements. The gas is
i then passed through a second packed section 115 which is washed
¦ with recirculating wash liquid and makeup water from nozzles 116
¦ to remove any entrained liquor containing TRS and solids. The
¦ sections shown in FIG. 2 are inclined at an angle of about 8-13
from the vertical in the direction in which the gas is moving.
¦ Such a construction is not critical but helps to prevent maldis-
I tribution of the liquor in the packing and thus insures full use
¦ of the packed section. The scrubbing liquor and washing liqu~d
¦ from sections 111 and 115, respectively, together with particu-
¦ lates, are drained to the bottom of the respective sections
through packing support gratings which are of such size that the
packing is supported while the liquid and suspended particulates
pass through and into collection sumps 108 and 117 respectively.
Pumps 118 and 119 are used to recirculate the scrubbing liquar
and washing liquid respectively. If desired, a single collection
sump below the packed sections and venturi can replace sumps
108 and 117 and the liquor collected in the single sump can be
recirculated by one or more pumps. Where two sumps are employed
as shown, they can ~e separated by an overflow weir 120 whereby
excess recirculating liquid, including fresh makeup water, can
flow into sump 108. By this means, the concentration of salts
and solids in the wash liquid in sump 117 can be maintained at
¦ a lowcr concentration than in thc liquor in sump 108.
Il
pa~le l O
106767~
To replace liquids lost with tlle gas and withdrawn
with slipstream 121, and to maintain thc desired concentration
of carbon ancl al}:~]i during use, ail as more fully explained
I)ereina~t~r, rr~sl~ ma~eup water i:; s~ ]led at 122, concentrated
caustic is added at 123, and carbon slurry is added at 124.
Also as more fully explained hereinafter, activated carbon in
the liquor slurry in sump 108 is aerated through submerged
nozzles 125 within the sump and fed at 126 through a compressor
(not shown).
Advantageously, after leaving the scrubbing section
115, the gas stream is passed through an open drainage zone 127
to allow drippage of entrained water droplets followed by a
demisting chamber 128. The demisting chamber is packed with
any suitable packing material, preferably the same material
used to pack the scrubbers. ~ subsequent ~emisting chamber .
may also be employed. The treated gas SO from the second
enclosure defined by baffles 105 and the walls of housing 107,
is~substantially free of particulates larger than about 0.1
micron.
As shown in FIG. 2, a single pump 109 can be used
to recirculate liquor for the baffle sprays 106 and the venturi
quench 102. As a further pre-treatment, prior to the venturi
- and baffles, the gas stream can optionally be passed through
washed fan 100 for additional increases in humidity and
turbulence and to improve the wetting of the particulates.
The fan c~n b washcd with a portion of one oE the
, 10
. ,,
,7~7~
recycled aqueous liquids, for example, the makeup water from
pump 119 as shown in FIG. 2.
FIG. 3 is a partially cut-away perspective view of
a ground level installation similar to FIG. Z and wherein like
parts have like numbers. Pumps 109 and 118 have been rearranged
to pumps 130 and 131. This figure illustrates that apparatus
according to this invention can be combined in a single compact
housing. FIGS. 4, 5 and 6 are described hereinafter.
DESCRIPTION OF_THE PREFERRED EMBODIMENT
General Description
In general, the present invention comprises the
following steps:
1) In a preliminary~step, the hot particulate-laden
gas containing a mixture of sulfur oxides, hydrogen sulfide,
and organic sulfur compounds is treated by conventional means
for the removal of particles larger than about 5 microns.
Such means are well known in the art and include a cyclone
separator, a spray tower, a venturi, an electrostatic preci-
pitator, and a tray column, either alone or in combination.
For example, the combination of a cyclone separator for the
preliminary removal of particles and a crossflow scrubbing
apparatus for the removal of very small particles is illustrated
in U.S. Patent No. 3,324,630. This step is optional since
the subsequent steps set forth below will remove large as well
as relatively small particles. However, if a significant quantity
of particles larger than about 10 microns are present, the
preliminary separation step will be more economical.
11
~ pa~c 12
1 ~0~767~3
¦ 2) Thc hot gas, preferably containing only particles
smaller than 10 microns in size together with various sulfur
contaminants, is n~xt subjected to a liquid quench immediately
prior to or simultaneous with its passa~e through the low energy
venturi. This treatment cools (althou~h maintaining the tempera-
ture above 150F) and moisturizes the gas to a point approaching
saturation conditions and introduces additional turbulence in the
gas. A wetted inlet fan can also be employed prior to the
venturi.
3) The gas is next passed through a liquor-washed
baffle system. This further cools (although still maintaining the
saturation temperature above 150F) and moisturizes the gas to
substantial saturation and also creates additional mixing in the
gas stream.
j ~) On leaving the baffle system the substantially
¦saturated gas is at a temperature above about 150F and these
conditions have promoted rapid nucleation among particles down to
an initial size of about 0.1 microns or less.
5) The gas stream is next passed through one or a
plurality of packed scrubbing beds in crossflow contact with a
scrubbing liquor. The preferred scrubbing liquor for this inven-
tion comprises an aqueous, alkaline suspension or slurry of
activated carbon as more fully described hereinafter.
6) The gas stream is recovered from the scrubber unit
essentially free of entrained particulate matter larger than about
0.30 microns and essentially free of sulfur compounds, the TRS
concentration being generally less than about 5 PPM. The gas may
then be exhausted to the atmosphere or further treated as follows.
7) Optionally, before discharge to the atmosphere, the
residual TRS in the gas stream can be further reduced by a second
oxidation and scrubbing step, described and illustrated
hereinafter, and/or the gas can be passed through a further
I 12 ~ ~,
11 "-,;,
1~ p~J~
10~7~7~3 I
packcd croc;s~low ;cction washcd with cool water to recover hcat
and furth~r r~duc~ p~rticulates. Ther~after, if desired, the
c3as can be ~a~scd through a suitable d~mistin~ chamber to remove
entrained droplcts of liquid, for example, a unit packed with
the same matcrial as the scrubbcrs but which is not washed with
any liquid.
Although the nucleation mechanism for fine particulates
is not thoroughly understood, it is believed to involve conden-
: sation of moisture on the fine particles and their agglomeration
by collision with and bonding to other such particles, thereby
increasing their effective size. Fine particulates are also
thought to have a surface electrostatic charge by virtue of
their high surface to mass ratio; Such charges are believed to
¦assist in the nucleation process.
!5 Adiabatic or isoenthalpic nucleation as herein dis-
closed and as disclosed in U~S. Patent Mo. 3,957,464 (May 18,
1976)/ is a function of essentially three variables, the
moisture content of the gas, the turbulence of the gas, and the
Itemperature of the gas. Thus it has been found that adiabatic
¦nucleation is not effective below abou~ i50 saturation tempera-
ture and that higher gas saturation temperatures compensate, in
part, for a lesser degree of turbulence in the gas and vice
versa. An increase in turbulence in the incoming gas, to a
Reynolds number of at least 3000, and pre~erably of at least
10,000 or more at the time of coolin~ to saturation is necessary.
With higher satuxation gas temperatures, either the venturi or
baffles, or both, can in some applications be omitted, although
both are preferred. Thus, where the incomin~ gas has a
saturation
:', . .' . ' ',
, .' '' " ' '',
: ~3
. ,,1.
. .
10~i7~78
temperature o~` ahout 190F -to 2]2 F, the venturi can be omitted.
At close to 212 F saturatlon temperature, both the venturi and
baffles can be omitted and a series of water jets employed. For
a given set of operating conditions, the turbulence of the gas
can be varied experimentally to optimize results. While it is
technically feasible to raise the saturation temperature of an -
incoming gas stream to a point requiring minimum turbulence, the
cost of doing so is ordinarily prohibitive. Turbulence, however,
can be increased comparatively inexpensively.
The packing elements or units that operate most satis-
factorily in the process and apparatus of this invention are
disclosed in applicant's U.S. Patent Nos. 2,867~425 and 3,324,630 and
are available commercially under the trademark "Tellerettes".
"Tellerettes" provide a filamentous packing having little con-
tinuous extensive surface and about 80-85% free volume therein;
the packing consisting of randomly arranged, interlocked tower
packing units, the units being made up of approximately circular,
integrally connected filament portions having their axes
approximately tangent to a circle at approximately evenly spaced
points therearound, the number of such spaced approximately
circular portions being from 6 to 12 and the diameter of such
circle being approximately equal to the diameter of one of such
approximately circular filament portions plus the diameter of
a smaller circle whose circumference is not less than the cross-
sectional dimension of the filament portion in the direction ofits axis times the number of such filament portions and not
greater than the circumference of one of such approximately
circular filament portions. Such packing units are hereinafter
referred to in the description and claims as "toroidal elements".
14
7678
I`he preferred scrubbing liquor for this invention is
alkaline aqueous slurry of activated carbon having a particle
size range perferably in the range of 0.05-10 microns and a
pH of about 8-13, more preferably 8-9.5, and most preferably
S about 9.0-9.3. The alkaline material in the scrubbing liquid
may be soluble sodium or potassium salt such as sodium hydroxide,
sodium carbonate, or the like or a relatively insoluble alkaline
earth metal salt such as lime or calcium carbonate in slurry
form. Sodium hydroxide is preferred.
The removal of S0z and TRS by the scrubbing liquor is
based on sorption and chemical reaction with hydroxide and oxygen.
S2 is converted to sulfates and TRS to oxidized sulfur compounds.
H2S for example is converted at least in part to Na2S203. Such
compounds are not volatile and can be recirculated in the
scrubbing liquor as dissolved or suspended salts. In addition
to the oxidized materials, the scrubbed particulates, principally
carbonates and sulfates of sodium, recirculate with the scrubbing
liquor.
Maximum recirculation of scrubbing liquor is an impor-
tant part of the present invention for reasons of cost andefficiency. With prior art processes the highest solids or
non-volatile content, i.e. the content of materials which are
essentially non-volatile at 212 F, that can be recirculated is
about 15% by weight. With the present process, however, the
non-volatile content may be as high as 25% and is preferably
in the range of 20-25% by weight. The crossflow scrubber of
this invention is stable at such high content.
Crossflow scrubbing has other important advantages in
the present invention. The ratio of scrubbing liquor to gas flow
rates can be varied along the depth of the packing, i.e. in the
7~78
direction of gas flow, as can the size of the packing elements.
Also different liquors of different composition or concentration
can be employed and recirculated. Preferably, higher flow rates
of the same scrubbing liquor are employed in upstream portions
of the packing where the S02 and TRS concentrations in the gas
are highest. Thus the ratio of alkali (and oxygen) to S02 and
TRS (and acid particulates such as NaHS04) concentrations in
the gas can be varied with the depth of packing. For example,
in FIG. 2, the valves controlling nozzles 112, 113 and 114 can
be adjusted to provide a high flow rate through nozzle 112, a
lower rate through nozzle 113, and still a lower rate through
nozzle 114. Under some conditions it has been found that, based
on the same total flow rate, such a distribution of scrubbing
liquor will be more efficient than an even distribution.
Similarly, it is sometimes desirable to employ larger packing
elements, e.g. 2 inch toroidal elements, in upstream portlons
of the packing and smaller elements, e.g. 1 inch toroidal elements
in downstream portions.
Sufficient alkali and carbon are required for efficient
reaction and removal of contaminants b~t excess should be avoided
for economy and to limit corrosion. Alkaline pH isnecessary
but the pH should be below about 9.5, and preferably 9.3, to ~;
avoid reaction with C02. With well-oxygenated, activated carbon,
a carbon content between about 0.03% and 0.20% by weight is
suitable and about 0.05% to 0.15% is preferred. These values
are lower, for given removal efficiency, with the present
invention than with prior processes because the scrubbing liquor
flow in the crossflow scrubber is laminar over the packing,
rather than turbulent. With laminar flow it is believed that the
16
, :
1067~78 pn~Je 17
suspcnded c~r~on migrates to the surface of the flowing liquor
and concentrates in the most active portion of the scrubber
liquor, that is, the portion in contact witll the gas. Below
about 200 PPM of TI~S in the gas, it has been found that a bulk
concentration of carbon in the weight range of about 0.03% to
0.07%is sufficient and above 200 PPM TRS, a range of about
0.0~Oto 0.15%is sufficient. Thus a carbon concentration range
between about 0.0~ to about 0.20~ by weight is preferred, the
particular value selected being a function of operating conditions
and TRS inlet concentration in the gas.
To maintain the non-volatile concentration in the
recirculated scrubbing liquor, a slipstream of liquor is bled
off and returned for processing to the material balance of the
pulp process. The high non-volatile concentration in the slip-
stream permitted by this invention is advantageous because a
minimum of carbon and unreacted alkali are thereby withdrawn with
the slipstream and less heat is required to remove water for
concéntrating the salts recovered in the slipstream. Fresh
makeup water and fresh alkali and carbon are added as required
to maintain pH and carbon concentration in the scrubbing liquor.
For the reasons given above, the consumption of alkali
and carbon in the present invention are low, generally in the
range of 0.3 to 0.6 pounds carbon and about 9 to 25 pounds of
alkali, measured as NaOI~, per ton of air dried pulp processed,
depending on the specific process conditions and control, and
the type of wood being pulped. These relatively low values are
important since such consumption is estimated to constitute the .
lar~est single item of cost in operating the process, including
page 18
~067ti78
amorti~ation of equipment. Properly operated, it is estimated
that the econonlic valuc of recovered salts returned to the pulp-
ing proc~ss can excc~d tllc total cost of operating the flue gas
¦ treatincJ process o~ this invention.
~rlle presen~ invention also has a low cost for power
and heat since the nucleation step requires low power and
essentlally no heat, while the scrubbing step preferably is
operated without significant cooling of either the gas or
scrubbing liquor, except incidentally in withdrawing of slip-
stream and adding of makeup materials. Crossflow scrubbing
also has an islherently low pressure drop for the gas such that
the entire process can be operated with a gas pressure drop
below about 30 inches of water, and typically less. Thus the
entire process is substantially adiabatic throughout and, so
operated, can reduce the particulates in the exhaust gas to about
0.03gr/sdcf. If further reduction is desired, the gas can be
exposed to a cooling liquid, either the scrubbing liquor itself
as shown in U.S. Patent No. 3,324,630, or fresh makeup water
as described herein, in either the whole of the packing of the
¦ scrubber, a portion thereof, or a separate packed section. By
such cooling, where desired, particulates can be further reduced
¦ to about 0.01 ~r/sdcf.
¦ The cross sectional area of the packed scrubber is
¦ chosen to accommodate the flow rate of gas to be treated and the
depth of packing, with respect to the direction of flow of the
gas, is chosen to provide the required removai of contaminants
to the extellt desired, greater depth providing increased
r~moval within the limits of the process. The required depth
pJ-JC ~9
1067~7~3
can be provide(l in continuous or separat~d sections. Scrubbing
liquor flow ratcs are chosen to maintain laminar liquid flow
over tIle slIrfacc of tI~c ~cking, aI-cI can be varied along the
deL~tIl of ~acking as describcd.
The following examples further illustrate the present
invention.
EXA;IPLE I
A series of tests were performed in an integrated
recovery apparatus as illustrated in FIG. 2 with flue gases
from a Kraft recovery process as illustrated in FIG. 1. Gas and~
process operating conditions are given in TABLE 1. The pressure
drop of the gas in the venturi was in the range between 4 and 10
inches of water, and in the total scrubber between 7 and 13
inches of water. The depth of the packing was about 5 feet
and the scrubbing liquor flow rate was varied along the depth
to provide greater flow upstream of the gas than downstream.
The system was found to be capable of a 2:1 turndown,
providing desirable flexibility of operation, and was relatively
insensitive to variations in liquid and gas flow rates. During
¦ testing, including operation 24 hours per day 7 days per week,
¦ no solids build up, no increase in pressure drop, and no adverse
I conditions such as undue foaming were observed.
1~ 3i0~7~78 P~3~ ~0
'r~131.1` 1 I
_ __~.
Gas Conclitions Inlet Outlet
_ . __ _
Gas Flow 235,000 200,000
(acfm)
_ _
l Temp. 300 163
l (~)
, .-
COIlCj SOxl 50-150 5-10
. . . -I
Conc. Particulates 1.5 0.02-0.06
(gr/sdcf)
.
TRS 600 3-5
(PPM)
, . .
1 - SOx is used to denote mixed sulfur oxides, predominantly SO2.
i
.
Scrubbing Liquid Inlet Outlet
. _ I
! Liquid Flow 3760 3713
, (gpm)
(F) 167 167
, Venturi quench liquid - 2200 gpm at 163F
! Baffle wash liquld 700 gpm '
~lake-up water - 50 gpm
~lake-up NaOI{ - 200-1000 lbs./hr.
! Make-up carbon - 5-15 lbs./hr.
i Air for oxygenation - approx. 1500 cfm
Recycle liquid: 20 gpm
22~ soli~s
i 0.1~ carbon
l Pl~ 9.3
i .
! acfm - actual cubic fcet p~r minute ,
Pl~l - Parts per millio1l 1
rjscdf - ~3rains per stand~rd dry cubic foot of gas
qpm - ~allol~s per minut~
1~RX ~ Pdllc~(l s~
_ __ _ _ _ _ _ _
~0
I h.
.. ........... _.__.
I pa~e 21
7~78
I~X~MI'I.~ II
Tl~e purposc of this cxanlplc was to comparc the process
¦of this invention with that taught by U.S. Patent No. 3,701,824,
l¦and in particlllar, to compare the efficiel-cy of a turbulcnt
¦ cantactor with the crossflow'scrubbing process of this invention
l at low levels of TRS emissions. The data for this example were
¦ obtained from tests at TRS levels of about 10-100 PPM using two
crossflow scrubbers and one turbulent contactor having the
l following characteristics:
~ TABLE 2
¦ Recovery Unit ~PCarbon Slurry - wt.-~
II Crossflow Scrubber 10 0.03-0.06
¦¦ Turbulent Contactor 16 0.5
,The results of these tests were plotted,on the basis
of efficiency (on a logarithmic scale) against TRS concentration
as shown in FIG. 4 wherein the solid curve represents the cross-
flow data and the broken curve the turbulent contactor data.
These tests demonstrate the superior efficiency of the cross10w
scrubber in removal of TRS emissions despite a ten-fold reduction
in the concentration of carbon in the slurry. Furthermore, these
data show tllat'the crossflow scrubbers operated at about a
30%'less pressure drop, therefore requiring less power than the
turbulent contactor.
EX~MPL~ III
The purpose of this example is to,demonstrate the
variation of caustic consumption and tllermal requiremcnts at
varying concentrations of dissolved solids (non-volatiles)
in the recycle scrubbing liquor in the stable crossflow scrubber
of this invention. The data
pa~e 22
i J.0ti7678
¦Iwas obtainc~ ~rom a llit3h emi!;sion boilcr having the followiny
¦¦characteristics:
O~)cratincJ Lcvcl - G00 Tl~D
¦ TRS - 500 PPM a~.
I~arl:i culate - 1. 5 c3r/sdcf
l Gas Flow - 200,000 acfm 160F Sat.
I The results are shown in Table 3 below:
T~BL~ 3
Recycle Liquor - Slip- UnreactedThermal Load
Dissolved Solids stream NaOH loss For Conc. to
(~ Concentration) rate (lb/ton 50% Solids
(GPM)* of pulp) (BTU/}lr.)
90.5 2642.8 x 106
1 10 47.6 13.721.5 x 106
27.0 7.811.1 x 106
19.1 - 5.57.2 x 106
?5 - 14.3 4.14.8 x 106
* Required to maintain solids concentration
EXAMPLE IV
This example demonstrates the effectiveness of this
invention in particulate removal, the variation of effectiveness
with gas saturation temperature, and the criticality of a gas
temperature above about 15 0F.
A series of tests were conducted at different gas satura-
tion temperatures between 155F to 172F,without cooling the
recycled scrubbing liquid, and with particulate loading ranging
from 0.17 to 0.54 gr/sdcf. The recovery boiler effluent was
pre-trcated with an electrostatic precipitator to remove larger
particles prior to entering the scrubbing unit. These results
22
~ pa~e 23
~L06~7678
are plotted in FIG. 5. The smothcd curve indicates a parti-
culate emission ranging from 0.050 gr/sdcf at an operating
temperature of 155F to 0.024 gr/sdcf at an operating temperature
¦of 172F, well within the proposed 1977 standard of 0.08 gr/sdcf.
EXAMPLE V
Tests similar to EXAMPLE IV were conducted with a
recovery boiler effluent gas pre-treated in a direct contact
evaporator. Duct thermal loss prevented conducting tests at
adiabatic temperatures above 162F. However, with inlet loadings
ranging from 0.8 to 3.0 gr/sdcf and with the scrubber system
operating at 16 to 19 inches of waterl particulate emissions
were reduced to 0.11 gr/sdcf. The particles from the evaporation
were found to have hydrophobic coatings; therefore, to accelerate
the initial wetting of these particles, additional turbulence
was induced in the gas prior to the scrubber. ~1ith added
turbulence prior to scrubbing, particulate emissions were
reduced to the order of 0.03 to 0.04 gr/sdcf, again well within
proposed 1977 standards.
By means of the process steps and apparatus illustrated
and described above, the treated flue gases will be at a
saturated temperature above about 150F and have a low con-
centration of particulates and TRS, suitable for exhausting in
compliance with existing governmental regulations. However,
the treated gases contain valuable heat. Also, it may be desirabl
¦ in some cases to reduce TRS and/or particulates still further.
¦ Optional steps to recover heat and/or further reduce TRS and
I particulates are described below.
'~3
I page 24
~Of~7~78
Wastc heat may be recovered by passing it in heat
exchange relation to a cooling fluid. In the present invention,
¦it is both convcnient and advantageous to recover heat by
Ipassin~ thc gas in crossflow contact with another packed section
of scrubber w~shed with clean, or relatively clean, cool water.
The additional scrubber section or enclosure can~be provided in
the same housing as the other sections and the heated watex can
be sent to the pulp process for utilization. The thus heated
water, after extraction of its heat to re-cool it, can be -
` 10 ~recirculated if desired. Compared with conventional heat-
exchangers or co-current or counter-current towers, the cross-
Iflow recovery unit is smaller, cheaper, less subject to
¦¦corrosion and has a lower pressure-drop and power requirement.
ilAlso, the water can be heated to approximately the temperature
¦ of the inlet gases to the section, unlike conventional
exchanges and co-current towers. Further, the sump below the.
unit for collecting the heated water can be segregated into
two or more portions along the direction of gas flow to provide
Il water of increasing purity, downstream portions being less
contaminated than upstream portions. The cool water will
remove addition particulates from the gas and become contaminated
thereby, especially if the water is recirculated.
It has been further discovered that the TRS in the
treated flue gases, generally below about 5 PPM, can be further
reduced by a second oxidative treatment by means of an
oxidizing agent more powerful than oxygen. The products of this
second oxidation, together with any excess oxidizing agent,
are scrubbed from the gas in an additional crossflow scrubber
:106'7671~ page 25
section with a recirculating alkaline scrubbing liquid similar
to the scrubbing liquor employed in the previous sections, but
omitting activated carbon.
Preferably, the oxidizing agent means employed in the
second oxidation is a chlorine-containing gas, such as C12 ox
ClO2, which is often employed in bleaching operations in pulp
processing. This gas can be mixed with the previously treated
flue gases in advance of the additional scrubbing section. A
slip-stream of the recirculating scrubbing liquid can be sent
to the pulp bleaching operations for use, especially where the
oxidizing agent employed is the same as that used in the bleachin~
process. Alternatively, bleaching agent means such as an alkali
metal, preferably sodium or potassium, or alkaline earth metal,
hypochlorite, permanganate or chromate; or chromic acid can be
dissolved or dispersed in the alkaline scrubbing liquid employed
in the additional scrubblng section.
1 Further oxidative reduction of TRS concentration and
¦~ the recovery of heat are both illustrated in FIG. 6. Flue gases
1l Si are fed by means of fan 100' into venturi 101' having a
~ diamond-shaped insert 104'. Thereafter the gases S are passed q
¦~ through baffles 105' which in this embodiment comprise a crossflow
section packed with elements, preferably toroidal elements, which
Ii are larger, e.g. three inches in diameter, than the elements
¦l employed in succeeding sections. In passing through ventuxi 101'
!l and baffles 105l, the tuxbulence of the flue gases is increased
and they are cooled to saturation, causing nucleated particulates
to form.
Following passage through baffles 105', the flue gases
Il pass through scrubbing sections 111' and 111" packed with
1, toroidal elements, e.g. two inch and one inch elements, respec-
I tively. Sections 111' and 111" are washed with an alkaline
., ~ ,
. . .
page 26
~0~7~78
aqucous liquor conta1nin~ ~xygenated, activated carbon as more
fully described in connection with thc preceding embodiment.
The scrubbinq liquor is collected and recirculated from a single
sump 108' and no separate washing section (115 of FIG. 2) is
employed. I.iquor from sump 108' is circulated by means of
pumps 201, 202 and 203 to sprays ],02' for the venturi 101',
spray 204 for baffles 105', sprays 205 and 206 for scrubbing
sections 111' and 111", and spray 207 for the blades of fan 100'.
To maintain liquor concentration, a slipstream 121' is returned
1 to the pulp process, and fresh caustic and carbon are added as
needed at 123' and 124', respectively. Compressed air from a
compres.sor (not shown) is introduced at 126' to nozzles 125'
submerged in scrubbing liquor in sump 108'. '
The flue gases exiting from primary oxidizing scrubbers
lll' and 111" will generally contain residual TRS, less than
about 5 PPM, and a low concentration of fine particulates.
To further reduce these concentrations, an oxidizing gas more
powerful than oxygen is fed at 210 into the space 211 between
scrubbing sections 111" and 212. Preferably, this gas is chlorin~ ,
¦ which mixes with the flue gas and causes further oxidation
of the residual TRS. Sufficient chlorine should be employed for~
oxidation, but the required amounts are small in view of the
low concentrations of TRS being treated.
Following admixture and treatment with oxidizing gas,
¦ the flue gases are passed through another crossflow scrubbing
¦ section 212, also containing packing, preferably one inch
toroidal elements. Section 212 is washed with scrubbing liquid
introduced at 213 and drains into sump 214 from whence it is
recirculated by means of pump 215. This scrubbing liquid is
preferably an aqueous caustic solution with the same p~l and
otherwise simi.lar ~o the li uor in Sull~p 10~', cxcept that it
~ . ~ .
page 27
~;7678
contains no ~ctivated carbon. Excess oxidizin~ gas and the
l products o~ oxidation are scru~bcd from ~hc gas in section 212.
¦¦ A slipstre~m 216 of the liquicl, including scrubbcd particulates,
I! the products of oxidation, and excess chlorine, is bled off and
¦ returned to the bleaching portion of the pulp process, and
additional concentrated caustic added at 217 to maintain pH
and volu~e.
The twice-oxidized flue gas exi~ing from section 212
! is then passed through another crossflow scrubbing section 220,
!l also filled with packing material, preferably one inch toroidal
¦ elementsO Section 220 serves as the convenient and efficient
¦ heat exchange chamber in which the hot incoming gases contact
¦ a cool fluid, either fresh cool water or recycled cool water
¦ from the pulp facility, which is sprayed into the top of the
section by means of nozzles 221 and which drains and collects
by gravity into sump 222. The hot water collected in sump 222
is withdrawn by means of pump 223 to the main pulp process for
recovery of heat via pipe 224. A portion of the hot water is
~ also pumped by means of pipe 225 to the inlet duct 226 for the
flue gases in advance of the fan lO0'. Such relatively pure
water for cooling and moisturizing the incoming gases in advance
of the fan is preferable to the liquor from sump 108' to avoid
the collection of precipitated solids on the walls of the
¦ incoming duct. Such collection is not normally a problem with
¦ respect to components of the apparatus downstream of the fan.
An emer~ency inlet plpe 227 is also provided to admit cooling
water to quench surges of unduly hot incoming gas.
¦ As previously indicated, cooling the gases in chamber
¦ 220 will remove additional residual particulates and suspended
¦ liquids from the gas. The concentrations thereof will often be
I
~ 7
page 28
1067678
too s~all to cause problems when fresh cooling water is employed,
~ut may build up with time where the cooling water is recycled.
Under such conditions, or where relatively pure water is otherwis~
desir~d, the sump 222 may be segregated into two or more sumps
of increasing purity downstream in the direction of gas movement,
downstream sumps providing increasingly pure water for such use.
Also, the cooling water flow rate can be adjusted to provide
any desired outlet temperature for the heated water up to
approximately the temperature of the gases exiting from section
212. Such heat recovery is an important part in the total
economics of the process and can reduce the cost of operation
considerably. In one design for treating the flue gases exiting
¦ from a 600-ton per day pulp mill, it has been found that 1.6 x
108 BTU!HR can be recovered in the form of hot water at a
temperature approximating saturation temperature of the flue
gases, for example 160F. .
While FIG. 6 illustrates the optional secondary
oxidative treatment by means of an oxidizing gas, other
suitable nongaseous oxidizing agents may be added to the liquid
circulating through scrubbing section 212 through spray nozzles
213 and sump 214. Gases containing chlorine are preferred since
they are effective and conveniently used, and since these are
the materials commonly employed in bleaching portions of pulp
processes. Excess gas, recovered in the scrubbing liquid! can
be sent to such bleaching operations for use.
EXAMPLE VI
A typical design for the integrated recovery apparatus
¦ illustrated in FIG. 6, with flue gases from a pulp process as
illustrated in FIG. l, has the operating conditions as shown
in TABLE II be]ow, wherein the terms employed have the units
and definitions stated in TABLE I.
~8
10~i7678 page 29
,' ' ' . I
TABLE II
; Gas Conditions: Inlet (si) Outlet (So
, _
Flow, ACFM 275,000 148,800
~:Temp., dry bulb 300F. 100F.
~wet bulb 160F. 100F.
: Concentration, SOx
Particulates 1.5 0.04
TRS 600 2 or less
Treating Materials: Flow Rate:
,
: Sump 108 (at about 163F.): :
Fan Wash (207) 150 gpm
Venturi spray, top 1750 gpm
Venturi insert (104') spray 750 gpm
Baffle spray (204) 825 gpm
Scrubbing spray (205) 3735 gpm
; Scrubbing spray (206) 3465 gpm i
; Slipstream (121') 24 gpm at about 20%
' non-volatiles
Make-up carbon (124') 5-20 lbs/hr
NaOH make-up (123') - 250-1200 lbs/hr
, Compressed air (126') about 2000 acfm at 10 PSIG
Chlorine gas (210) 2-10 acfm
Sumps 214 and 222:
Scrubbing liquid (213) 400 gpm at about 163F.
I Slipstream (216) 0.5 gpm
l NaOH make-up (217) 25-125 lbs/hr
Cooling Water (221) 3600 gpm at about 51F.
i Hot Water (224) . 3870 gpm at- about 156F.
I Hot Water Spray (225) 80 gpm
I ~
¦¦ lbs/hr - pounds per hour
PSIG - pounds per square inch gauge
¦l All gallons are U.S. measure
~ .
hile demister 128' is shown in FIG. 6 as the last
packed section in the process, it can precede heat exhange
section 220. Where clean water is required such position reversal
~has the advantage that entrained liquids with any impurities
icontained therein are removed in advance of heat recovery. Where
~1 !
29
, . ,
101~7~;78 p~ge 30
the units 128' and 220 are thus reversed, sump 222 can be divided
into two sumps, the first (in the direction of gas flow)
receiving the entrained liquid drainging from demister 128',
with a separate discharge, and the second receiving the heated
~ water as shown in FIG. 2, but omitting any demister drainage.
One form of crossflow heat-exhange apparatus is described in
l U.S. Patent No. 3,759,496.
In the foregoing description of concentrations of
activated carbon, the concentrations stated, and the comparisons
!,
with prior art given, were for activated carbon of low density,
made for example from wood bark. Activated carbons vary in
density from about 0.0~ to about 0.5. By low density as used
herein are meant those having density below about 0.2. Where
Il more dense forms of activated carbon are employed, for example
1 activated carbon made from coal, the carbon concentration should
be increased two to three times to compensate for its lower
1 surface area and greater tendency to settle in the slurry.
il Thus the preferred range for low density carbon is from 0.03
~I to about 0.2% by weight of slurry as stated. The upper limit
) for high density carbons should be increased to about 0.6~.
i A similar increase is required with prior art processes and the
I present invention has the advantage of lower requirements when
¦~ activated carbons of equal densities are compared. Higher
¦l amounts of carbon can be used but are not required and do not
1I provide benefits to warrant the increased cost.
I ~aving described the invention, what is claimed is:
~ i
.
,, .
